The utilization of cellulosic waste materials, such as cornstalks, sawdusts, straws, bagasse, and the like, has been the subject of strong interest recently, particularly with respect to utilization of such waste materials for developing alternate sources of fuels, feedstuffs, chemicals and other useful products.
Cellulosic materials include three principal components--cellulose, hemicellulose and lignin. Methods for extraction of hemicellulose have heretofore been suggested and/or utilized and such extracted hemicellulose can be utilized by many existing methods including hydrolysis, fermentation, pyrolysis, and the like.
Lignin has also been isolated from cellulosic materials and since it is higher in hydrogen and carbon and lower in oxygen content than cellulose and hemicellulose it has the highest fuel utility of the three. Isolated lignin can be burned to generate steam and electricity and can also be used to produce a number of useful products including vanillin, dimethylsulfoxide, dimethyl sulfide, and methyl mercaptan and catechol.
Recovery of cellulose and/or utilization of the same, as by hydrolysis to provide glucose, has presented a problem heretofore primarily due to the crystalline structure of the cellulose molecules and the presence therein of a lignin seal.
Attempts have been made to hydrolyze cellulose, and these attempts have included the use of acids or enzymes, but such attempts have not been completely successful, at least not in providing an economically attractive method that is capable of providing a satisfactorily high yield of glucose for the cellulose in such cellulosic materials.
The use of acids including sulfuric acid in hydrolysis of cellulosic materials to produce glucose has been known for many decades. In general, all previously reported processes of cellulose hydrolysis using sulfuric acid can be classified into two large groups; those using dilute sulfuric acid and those using concentrated sulfuric acid. The dilute acid processes typically involve a sulfuric acid solution containing somewhere from 5 to 50 grams of H.sub.2 SO.sub.4 in a kilogram of aqueous solution. At a temperature of somewhere from 100.degree. C. to 350.degree. C., cellulosic materials such as wood chips suspended in the dilute acid will be hydrolyzed to produce glucose from cellulose and five carbon sugars from hemicellulose. Since the reaction temperature is usually above the boiling point of the dilute acid, the hydrolysis reaction has to be carried out in pressurized reactors. At a high temperature, glucose and five carbon sugars will also be catalyzed by the acid to form furfural and its derivatives which often react further to form undesirable by-products. The dilute acid processes have thus not been generally successful in obtaining glucose from cellulose in high yields and at low costs. Typically, a dilute acid process gives a glucose yield of about 50% or less based upon available cellulose, hampered by the formation of useless by-products. The dilute acid processes have been well known and extensively studied by many researchers.
There are also a number of reports on processes of cellulose hydrolysis using concentrated sulfuric acid. For instance, there is a relatively recent report by Bose et al. (See Bharati Bose, T. R. Ingle and J. L. Bose, "Saccharification of Groundnut Shell Pulp with Sulfuric Acid", Indian Journal of Technology, Vol. II, September 1973, Pages 391-393) and an earlier report by Dunning and Lathrop (See J. W. Dunning and E. C. Lathrop, "The Saccharification of Agricultural Residue", Industrial and Engineering Chemistry, Vol. 37, 1945, Pages 24-29). In general, these processes involve adding concentrated sulfuric acid containing typically 750 grams of H.sub.2 SO.sub.4 in a kilogram of acid to a finely divided cellulosic material. After soaking and much blending and mixing, 8 to 10 volumes of water is added to dilute the acid. The mixture is then refluxed for a few hours at atmospheric pressure to produce glucose. Typically, 90% to 93% yield of glucose based upon available cellulose can be obtained by this method without much byproduct formation. The previously reported concentrated sulfuric acid processes, however, suffer from the fact that a large amount of acid is used. After the completion of the hydrolysis, both the acid and the glucose are dissolved in the same aqueous solution. An equally large amount of alkali (usually lime) must then be utilized to neutralize the acid before the sugars can be utilized as, for example, a carbon source in a yeast fermentation. In these processes, the problem of disposal of a large amount of calcium sulfate as a by-product must be faced in addition to the high costs for purchasing both the acid and the alkali.